feat(pwa): support asymmetric no-reply conversation flows

[Copilot]

Summary

The PWA assumed a strict request/reply cadence and kept user messages pending until a bot reply appeared. This change decouples delivery state from reply rendering so ack-only, empty-body, and timeout paths resolve cleanly without blocking follow-up sends or rendering empty assistant bubbles.

• Polling + acknowledgement handling

  • Treats explicit ack-style payloads (reply: null or status in the acknowledgement set) as processed when no newer reply exists.
  • Accepts empty response bodies in polling/response fetches and resolves the pending user message without requiring a bot bubble.
  • Preserves the existing “newer reply available” path so mixed ack/reply payloads still render the reply.

• Message lifecycle

  • Marks user messages as Sent immediately after the message POST succeeds.
  • Resolves no-reply/processed outcomes as Read.
  • Resolves polling timeout outcomes as Delivered, separating transport success from bot reply generation.

• Empty bubble prevention

  • Normalizes blank / whitespace-only transcripts to “no content”.
  • Skips assistant message insertion when neither transcript nor audio is present.

• Regression coverage

  • Adds focused frontend tests for:
    • ack-only status payloads
    • empty/no-reply response payloads
    • timeout-to-delivered transitions
    • threaded follow-up replies continuing to render correctly

if (isAcknowledgementOnlyPayload(payload) && !hasNewResponse) {
  resolvePendingConversation(conversation, elements, state, MESSAGE_STATUS_READ);
  return { nextDelayMs: normalizePollDelay(statusPayload.cacheTtlSeconds) };
}

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Final Checklist

• ADR Compliance: This PR adheres to all Accepted ADRs.
  • ADR-005: Poll Cloudflare Worker for response status instead of S3 directly
  • ADR-002: Use SQS long polling instead of WebSockets
• All Definition of Done criteria from the linked issue have been met.

• Fixes feat(pwa): decouple 1:1 message lock to support asymmetric conversation flows #97